1. Field of the Invention
The present invention relates generally to a method and apparatus for detecting defect or crack in an elongated metal strip, such as a steel belt. More specifically, the invention relates to a technology for discriminating defect or crack from intersection of two strips which are joined by way of welding.
2. Description of the Background Art
In apparatus employing an endless steel belt, such as a belt conveyer, a crack can be formed during operation of the apparatus. Once cracking occurs on the steel belt, breakage of the belt may subsequently occur to cause damage for associated components. Therefore, it is preferred to detect the occurrence of a crack in order to know the timing of maintenance of the steel belt. Conventionally, it is typical to monitor the condition of the steel belt by means of a non-contact type proximity switch. In the conventional technology, the proximity switch generates high frequency magnetic field to induce an eddy current on the steel. Magnitude of induced eddy current is variable depending upon the surface condition of the steel. Namely, when a crack is formed on the surface of the steel, magnitude of the induced eddy current fluctuates from that induced at the normal surface condition.
With such conventional crack detecting apparatus, a difficulty is encountered when the endless steel belt has a welded joining portion. Namely, the welded section has higher electric resistance than that in the general section of the steel belt. Therefore, the magnitude of induced eddy current at the welded portion becomes different from that of the normal surface. This brings erroneous judgement that the crack is caused on the steel belt.